There is an ongoing demand for efficient (high Lumens per Watt) lighting systems powered directly by an alternating current (AC) power mains (for example, 120VRMS, 60 Hz, or 230VRMS, 50 Hz). Examples include household and commercial indoor lighting, outdoor street lights, traffic lights, signage, etc. One example technology for efficient light emitters is Light Emitting Diodes (LED's).
FIG. 1A illustrates a lighting system 100 in which LED's 102 are connected in series and driven directly by a rectified AC supply voltage VRAC. The system may also include a current limiter or current regulator 104.
FIG. 1B illustrates example timing for the lighting system 100. Let VT be the threshold at which the supply voltage VRAC exceeds the forward-biased voltage of the entire series of LED's 102 plus the voltage drop across the current limiter 104. At time t0, the supply voltage VRAC starts increasing from zero. At time t1, the supply voltage VRAC exceeds the threshold VT and the LED's 102 emit light. At time t2 the supply voltage VRAC falls below the threshold VT and the LED's 102 stop emitting light. As a result, the LED's 102 are on only during the time period from t1−t2 (the shaded portion 106 of the supply voltage VRAC). Accordingly, light is emitted for only a fraction of the time, and the light flickers at twice the frequency of the AC power mains. If the peak of the supply voltage VRAC drops too far (for example, during a “brown-out”, or as a result of a dimming switch) then the lighting system 100 may fail to turn on.
FIG. 2 illustrates an alternative example of a lighting system 200 in which current for LED's is provided by electronic drivers. In the example of FIG. 2, a rectified AC supply voltage VRAC provides power to a plurality of driver/bypass circuits (204, 206, 208, 210) connected in series, and to a current limiter or current regulator 202. Each driver/bypass circuit (204, 206, 208, 210) drives one LED (212, 214, 216, 218). Each driver/bypass circuit (204, 206, 208, 210) includes a bypass switch that can bypass current around its LED. When the supply voltage (VRAC) exceeds a voltage sufficient to power LED 212 (and the current limiter or current regulator 202, and accounting for the series voltage drops of the bypass switches), driver/bypass circuit 204 turns on, opens its bypass switch, and drives its LED 212. As the supply voltage (VRAC) continues to increase, the driver/bypass circuits (206, 208, 210) sequentially turn on (and open their bypass switches) until all of the LED's are being driven. When the supply voltage (VRAC) decreases, the driver/bypass circuits (204, 206, 208, 210) sequentially turn off (and close their bypass switches). As a result, LED's start turning on at a relatively low voltage, and as the supply voltage (VRAC) increases, more LED's are driven and the overall intensity increases. As the supply voltage (VRAC) decreases, fewer LED's are driven and the overall intensity decreases.
There is an ongoing need for improved lighting systems.